Fuel oils are generally provided as products by suitably mixing a distillation residue and a distillate oil from petroleum with each other according to product specifications, and used as fuel oils for industrial boilers, heating furnaces and the like.
Residual oils generally comprise asphaltene, resins (malthene) and oil. A fuel oil containing such a residual oil causes various troubles such as clogging of fuel strainer and accumulation of sludge on the bottom of a fuel tank when the asphaltene is deposited as sludge. In an fuel oil good in storage stability, the asphaltene is dispersed as micelles in the resins and forms a sort of colloidal structure. In this case, the aromaticity of the base oil greatly affects stability. Insufficient aromaticity tends to aggregate and settle the asphaltene, resulting in formation of sludge.
In recent petroleum industry, it has been common to use deep drawing apparatus and fluid cat-crackers in atmospheric distillation, or secondary equipment such as thermal cracking apparatus and hydrocracking apparatus in order to cope with increased demand of gasoline to obtain more middle cut from crude oil. In fuel oils making use of residual oils, particularly, C-type fuel oil, this is attended more often than before with problems of reduced storage stability and filterability of fuel oil due to the formation of heavier residues and mixing of light cycle oil (LCO) therein. This reason is considered that the amount of cracked residue and visbroken residue used as a base for a fuel oil increase, and in particular, asphaltene and a saturated component in the residual oil contained in C-type fuel oil become increased, while the resins and aromatic component therein are decreased, so that the asphaltene forming micelles in the petroleum resin becomes deposited as sludge. Therefore, even when A-type fuel oil containing a great amount of the saturated component is blended with C-type fuel oil or the like, sludge is liable to form, resulting in a blend poor in mixing stability.
As dispersing agents for preventing the occurrence of asphaltene sludge in fuel oils, there have hitherto been used, for example, metal salts of sulfonic acid and naphthenic acid, long-chain alkyl dithiophosphates, surfactants such as higher fatty acid esters, and high-molecular weight compounds containing methacrylates and/or maleic acid groups. As a residual fuel oil additive for preventing the occurrence of sludge, it has recently been proposed to use a mixture a long-chain alkyl phosphate and an imidazoline derivative or a hydrolyzate thereof (Japanese Patent Application Laid-Open No. 23991/1988).
However, the conventionally known additives are still insufficient in respect of the effect to prevent the occurrence of sludge. There is thus a demand for the development of an additive capable of more improving the stability of fuel oils.